Windows including glass substrates with solar control coatings provided thereon are known in the art. Such windows may be used in the context of architectural windows, insulating glass (IG) window units, automotive windows, and/or the like.
Commonly owned US Patent Application Publication No. 2002/0.192474 discloses a heat treatable (HT) low-E coating, including a pair of silver layers and numerous dielectric layers. While this coating is certainly a good overall coating, usable in applications such as windshields and architectural windows, it is problematic in certain respects.
For example, Example 1 in 2002/0192474 has a visible transmission (TY or Tvis), measured monolithically, of about 69.2% before heat treatment (HT) and about 79.51 after FIT. Moreover, Example 1 of 2002/0192474 has a sheet resistance (Rs) of 4.30 before HT and 2.90 after HT (taking into account both IR reflecting silver layers). Thus, Example 1 in 2002/0192474 is characterized by a ratio of visible transmission to sheet resistance (i.e., ratio Tvis/Rs) of 16.1 before HT, and 27.4 after HT. Conventionally, this is a fairly high (good) ratio of visible transmission to sheet resistance compared to many other conventional coated articles.
It is known that if one wants to improve a coating's solar performance (e.g., infrared reflection), the thickness of the silver layer(s) can be increased in order to decrease the coating's sheet resistance. Thus, if one wants to improve a coating's solar performance by increasing its ability to reflect infrared (IR) rays or the like, one typically would increase the thickness of the IR blocking (or reflecting) silver layer(s). Unfortunately, increasing the thickness of the silver layer(s) causes visible transmission (TY or Tvis) to drop. Accordingly, in the past, it can be seen that when one sought to improve the solar performance of a coating in such a manner, it was at the expense of visible transmission. In other words, when solar performance was improved, visible transmission was sacrificed and decreased. Stated another way, it has been difficult to increase the ratio of visible transmission to sheet resistance (i.e. Tvis/Rs), especially if heat treatability and/or durability are to be provided. This is why many coatings that block (reflect and/or absorb) much IR radiation have rather low visible transmission.
An excellent way to characterize a coated article's ability to both allow high visible transmission and achieve good solar performance (e.g., IR reflection and/or absorption) is the coating's Tvis/Rs ratio. The higher the Tvis/Rs ratio, the better the combination of the coating's ability to both provide high visible transmission and achieve good solar performance.
As explained above, Example 1 in 2002/0192474 is characterized by a ratio of visible transmission to sheet resistance (i.e., Tvis/Rs) of 16.1 before HT, and 27.4 after HT, measured monolithically.
As another example, in U.S. Pat. No. 5,821,001 to Arbab, single silver Example 1 has a ratio Tvis/Rs of 10.7 before HT, and 19.5 after HT. Double silver Example 2 of the '001 patent has a ratio Tvis/Rs of 14.4 before HT, and 22.1 after HT.
As another example, the non-heat treatable version of Example 1 of U.S. Pat. No. 6,432,545 to Schicht relates to a single silver layer stack (not a double silver stack) having a ratio Tvis/Rs of 19.8 with no HT. The heat treatable version of Example 1 of U.S. Pat. No. 6,432,545 (which has a pre-HT Tvis of 70%) also relates to a single silver layer stack, but has a ratio Tvis/Rs of 16.7 before HT, and 28.8 after HT.
It can be seen from the above that commercially acceptable conventional heat treatable coatings cannot achieve very high Tvis/Rs ratios, thereby illustrating that their combined characteristic of visible transmission relative to sheet resistance (and solar performance) can be improved.
In the past, it has been theoretically possible to increase the Tvis/Rs ratio, but not in a commercially acceptable manner. For example, U.S. Pat. No. 4,786,783 alleges that a coated article therein has a rather high Tvis/Rs ratio (the 76.4% visible transmission alleged in this patent is suspect to some extent in view of the very thick silver layers in Example 2). However, even if one were to believe the data in the '783 patent, the coated articles therein are not commercially acceptable.
For example, Example 2 of the '783 patent can only achieve the alleged visible transmission of 76.4% by not including sufficient protective dielectric layer(s) or silver protecting layer(s). For example, Example 2 of the '783 patent has, inter alia, no protective contact layers (e.g., Ni, NiCr, Cr, NiCrOx, ZnO, Nb, or the like) between the bottom titanium oxide layer and silver layer to protect the silver during HT (protective contact layers reduce visible transmission, but protect the silver during HT). In other words, Example 2 of U.S. Pat. No. 4,786,783 could not survive heat treatment (e.g., heat bending, tempering, and/or heat strengthening) in a commercially acceptable manner. If Example 2 of the '783 patent was heat treated, the sheet resistance would effectively disappear because the silver layer(s) would heavily oxidize and be essentially destroyed, thereby leading to unacceptable optical properties such as massive (very high) haze, very large ΔE* values, and unacceptable coloration. For example, because Example 2 of the '783 patent does not use sufficient layer(s) to protect the silver during HT, the coated article would have very high ΔE* values (glass side reflective and transmissive) due to heat treatment; ΔE* over 10.0 and likely approaching 20.0 or more (for a detailed discussion on the meaning of ΔE*, see U.S. Pat. Nos. 6,495,263 and/or 6,475,626, both of which are hereby incorporated herein by reference). Clearly, this is not a commercially acceptable heat treatable product.
Thus, in certain instances, it may be desirable to: (a) increase visible transmission without sacrificing solar performance, (b) improve solar performance without sacrificing visible transmission, and/or (c) improve both solar performance and visible transmission. In other words, it may sometimes be desirable if the Tvis/Rs ratio could be increased, in a coating that may be heat treated in a commercially acceptable manner. For example, it may be desirable to achieve (a), (b) and/or (c) while simultaneously being able to keep the ΔE* value (class side reflective and/or transmissive) due to HT below 8.0 or the like.